GPRS (General Packet Radio Service), that is currently being specified by ETSI as a service for GSM phase 2+, provides packet switched connections throughout the PLMN (Public Land Mobile Network). The packet switching is performed between the nodes in the network and on the radio interface.
The most critical part of the transmission is between an MS (mobile station) and the SGSN (Serving GPRS Support Node). The unpredicatable change of the position of an MS has led to a wish for quite frequent update messages from the MS towards the network. These frequent messages cause a load on both the air interface and internally within the SGSN.
The protocol stack for GPRS between MS and SGSN is sketched in FIG. 1.
For the mobile telephony system to know the position of a mobile station and whether the mobile station really has radio contact with the system, periodic messages from the mobile stations are specified. The time period between the periodic messages is broadcasted from the network towards the mobile stations. With a scenario of 100.000 attached subscribers and 15 minutes updating interval, a considerable number of routing update procedures must be performed every minute. This will consume a significant part of the total processor capacity for the SGSN.
The current procedure will be discussed in the following:
Routing Area Update Request messages shall be sent unciphered, since in the inter-SGSN routing area update case the new SGSN shall be able to process the request.
The intra SGSN routing area update procedure is illustrated in FIG. 2. The example assumes that the MSC/VLR is not changed during the routing update. Each step is explained in the list illustrated in FIG. 2.
1) The MS sends a Routing Area Update Request (TLLI, new CI, old RAI, new RAI) to the SGSN. CI is added in the BSS to reduce the load on the radio interface. PA1 2) Security functions may be executed. PA1 3) The SGSN validates the MS's presence in the new RA. If, due to regional, national or international restrictions, the MS is not allowed to attach in the RA or subscription checking fails, then the SGSN rejects the routing update with an appropriate cause. If all checks are successful then the SGSN updates the MM context for the MS. A new TLLI may be allocated. A Routing Area Update Accept (TLLI, Cause) is returned to the MS. PA1 4) If TLLI was changed, the MS acknowledges the new TLLI with Routing Area Update Complete (TLLI). If the routing area update procedure fails a maximum allowable number of times, or if the Routing Area Update Accept Cause parameter indicates a reject, the MS shall enter IDLE state.
The working assumption does not take into account the differentiation in nature of the normal routing area update procedure (always in an unknown area) and periodic updates (always in a known area).